This steel tube allows the lower engine mounting bolt to cross through the engine housing. Every restorer should inspect this tube carefully and disassemble the conical rubber bushings for that purpose.
These rubbers should be replaced, because they are becoming harder with the years and are then unable to cope with the engines vibrations.
I was searching for an oil leak at my first Bergmeister, changing the gaskets and sealings at the oil sump, cylinder and so on several times, just to find out that the steel tube of the engine mounting bolt was corroded though and let some oil pass. The oil found its way to the outside and leaked in direction oil sump. We don’t have to wonder about this “rust phenomenon”, because with the rubbers becoming hard, water can migrate from the outside though the bushings, doing its corrosive work in the steel tube.
Our IG-colleague Schinagl pointed out something special at his engine housing (see picture below), what we had not known till then. He was so kind to remove the oil sump once again to take that photo for us (Thanx!).
In this housing, there is no steel tube anymore, but the cross through engine mounting housing is now integrated into the block casting. These housing version is very rare, only to be found in the latest production series.